High temperature thermal processes, for example, the generation of steam for the production of electricity in power plants utilizing fossil fuels, often create environmentally harmful by-products. These compounds, specifically nitrous oxides (NOx), have to be removed from the flue gases of the high temperature thermal processes before being discharged to the environment, for example before exiting a power plant and contacting the environment. The standard for removing NOx from flue gases is the selective catalytic reduction (SCR) process, where a reducing reagent, typically ammonia, is injected, mixed into the flue gas, and sent through a catalytic reaction chamber where the catalyst facilitates the reduction of NOx with the reducing agent to form elemental nitrogen and water. The catalyst, often referred to as a DeNOx catalyst, aids these reactions and is typically constructed of titanium dioxide containing the oxides of transition metals such as, for example, vanadium, molybdenum, and tungsten to act as catalytically active components. The catalyst is arranged on plates, in a honeycomb fashion or as a corrugated structure and placed parallel to the direction of flue gas flow. However, during operation, the catalyst undergoes a loss of activity and efficiency as a result of plugging with ash and deactivation of the active components from certain compounds contained in flue gas which are poisonous to the catalyst.
There is also a trend in the industry to find alternative fuel sources to fossil fuels. One such example is the use of bio-based fuels such as bio-residues and biomass which refer to living and recently dead biological material that can be used as fuel or for industrial production. Production of biomass as an alternative fuel source is a growing industry. Such materials may include plant matter or animal matter and biodegradable wastes that can be burnt as fuel and typically exclude organic material which has been transformed by geological processes into substances such as coal or petroleum.
However, the use of bio-based fuels such as biomass and bone meal used alone or in combination with fossil fuel present new challenges for catalyst regeneration. In addition to many of the flue gas constituents poisonous to the catalyst that typically affect the activity of a catalyst used in thermal process flue gas cleaning such as a power plant burning traditional fossil fuels, there are other compounds poisonous to the catalyst that present greater challenges for SCR catalyst regeneration. For example, the regeneration of phosphorous poisoned catalysts due to the deactivation of the catalysts presents a challenge.
According to an article entitled, “Deactivation Mechanisms of SCR Catalysts During the Co-Combustion of Bio-Residues” of J. Beck, S. Unterberger, K. Hein, the substitution of fossil fuels by bio-residues such as sewage sludge, waste wood or meat and bone meal is discussed as being of increasing interest for reduction of carbon dioxide (CO2) emissions. According to the article, during the co-combustion of these fuel mixes in existing coal-fired power plants, an increased deactivation of SCR-DeNOx catalysts is observed. The same effect can be observed when certain fossil fuels are burned or when other flue gases exposing a similar composition are treated by the SCR process. Analyses of deactivated catalyst samples indicate that the high concentration of phosphorus and sodium compounds as constituents of bio-residues has a significant influence on the deactivation rate of the catalyst. The article states that to determine the effect of these compounds, lab-scale experiments were carried out using doped synthetic flue gas, and in bench scale tests the behavior of phosphorus was investigated during the combustion of coal and phosphorus rich secondary fuel. Analyses of sub-bituminous Powder River Basin (PRB) coal also indicate the presence of organically bound alkali and phosphorus species. Recent operational experiences showed that in case of meat and bone meal (MBM) co-combustion in German utility boilers a share of 4 wt-% MBM leads already to a severe deactivation of the installed SCR-catalyst. Phosphorus and alkali compounds in the fuel were considered as primary deactivating elements.
In Table A, a typical elemental breakdown and the concentration percentages of many alternative bio-based fuels used in power plants are shown. For example, Table A sets forth sources that include wood, corn, sewage sludge, and meat and bone meal (MBM). As shown in Table A, phosphorous represents a high concentration in such fuels and accounts for a large percentage of what needs to be removed from the catalyst by regeneration of the SCR catalyst.
TABLE ABoneSewageMealWood*CornSludgeMBMAsh(Concen-(Concen-(Concen-(Concen-ElementAnalysistration %)tration %)tration %)tration %)SiliconSiO220-70 0.5-5 5.6-25.7—(Si)AluminumAl2O35-100.1-11.1-8.5—(Al)CalciumCaO2-300.1-1 1.4-42.9—(Ca)SodiumNa2O1-100.1-20.1-0.84-7(Na)PotassiumK2O2-15  10-300.3-1.61.5-4  (K)PhosphorousP2O51-5   10-601.2-4.425-40(P)*Includes pallet grindings, wood chips, mill residue, and clean wood waste
Thus, there is a need for a method for the regeneration of SCR catalyst that is particularly deactivated from phosphorous components in the flue gases from thermal processes such as power plants, especially in case of co-firing biomass instead of only coal.